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Science Under a Stadium
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20714 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
9 / 1992 |
2,251 Words |
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Jan McCoy
Jan McCoy is a science writer in Tucson, Arizona. |
This part spring, a visionary team of astronomers and engineers at the University of Arizona's Steward Observatory Mirror Laboratory cast the largest telescope mirror ever made in the United States. The 5-meter Hale Telescope of California's Mount Palomar had held the size record for nearly 60 years, but now takes second place to the newly cast mirror, which has a diameter of 6.5 meters.
This new mirror, which has a novel honeycomb interior, is the first of what is being considered the next generation of telescope mirrors. Using a unique spin-casting technology, solid chunks of glass are melted in a honey-comb-structure mirror mold built inside a giant, rotating furnace. When the glass, liquefies, the furnace begins spinning, creating centrifugal forces that sculpt a parabolic curve into the surface of the liquid glass. The curvature--which reduces grinding and polishing time--is captured by cooling the glass quickly. Mirrors cast in this manner maintain the rigidity and stability of traditional solid glass mirrors but are lightweight and adjust to changing temperatures.
The invention of this technology and the birth of the Steward Observatory Mirror Laboratory, colloquially called the Mirror Lab, date back to the 1970s, when astronomer and optical scientist Roger Angel was part of a University of Arizona-Smithsonian Institution team developing the Multiple Mirror Telescope (MMT) on Mount Hopkins in Arizona. Dedicated in 1979, the MMT has six 1.8-meter mirrors that work together as a single 4.5-meter telescope, making it the world's third-largest optical telescope.
"For many of us, the MMT was a real introduction to some of the possibilities of bigger telescopes," says Neville Woolf, a Steward Observatory astronomer who was acting director of the MMT during development and construction. "Roger clearly saw the need for bigger telescope mirrors; something new was needed." (The Mirror Lab is one of the research facilities of the Steward Observatory, which is headquartered at the University of Arizona in a separate building and includes a network of telescopes in southern Arizona.)
Ideas for making bigger telescope mirrors came and went. Some worked, some did not. One of Angel's ideas involved casting large mirrors in a lightweight, honeycomb structure. In 1980, he and Woolf wrote a paper outlining the idea for making 8-meter mirrors.
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